### How to Compare Two Dependent Proportions

In math class, we spend a lot of time learning fractions because they are so important in everyday life (e.g., budgeting, purchasing at the grocery store). Fractions are also used extensively in UX research (e.g., the fundamental completion rate is a fraction), typically expressed as percentages or proportions. Unfortunately, fractions are also hard to learn,

### Sample Sizes for Rating Scale Confidence Intervals

Sample size computations can seem like an art. Some assumptions are involved when computing sample sizes, but it should be more math than magic. A key ingredient needed to cook up a sample size estimate is the standard deviation. You need yeast to make bread, and you need a measure of variability to make an

### How to Use the Finite Population Correction

What is the impact if you sample a lot of your population in a survey? Many statistical calculations—for example, confidence intervals, statistical comparisons (e.g., the two-sample t-test), and their sample size estimates—assume that your sample is a tiny fraction of your population. But what if you have a relatively modest population size (e.g., IT decision-makers

### Sample Sizes for Comparing SUS Scores

Microsoft Word is a widely used word processing program, part of the Microsoft Office suite of programs. While its dominance has been challenged recently by Google Docs, Word still leads on the features list, providing many features that Google’s offering lacks. But adding features can also add to bloat, making common tasks harder as users

### Eight Laws of Statistics

Statistics doesn’t have a Magna Carta, constitution, or bill of rights to enumerate laws, guiding principles, or limits of power. There have been attempts to articulate credos for statistical practice. Two of the most enduring ones are based on the work by Robert P. Abelson, a former statistical professor at Yale. If Abelson wasn’t the

### Sample Sizes for a SUS Score

Despite its age and the availability of other UX measures such as the UX-Lite™ and SUPR-Q®, the ten-item System Usability Scale (SUS) is still a very popular measure. It’s used widely in benchmark tests of software products to generate an overall score of perceived usability. We regularly collect SUS scores for dozens of consumer and

### From Statistical to Practical Significance

Hypothesis testing is one of the most common frameworks for making decisions with data in both scientific and industrial contexts. But this statistical framework, formally called Null Hypothesis Statistics Testing (NHST), can be confusing (and controversial). In an earlier article, we showed how to use the core framework of statistical hypothesis testing: you start with